When a plurality of writing instruments is to be carried by a user, it has hitherto been the practice to place them in a box or case in order to keep them from becoming damaged or lost. Sometimes the use of a box or case is inconvenient because of bulkiness. In order to overcome this problem, others have proposed to connect the caps of several writing instruments together in a row either by molding them together in a unitary block, or detachably securing them together such as is disclosed in Japanese Utility Model Publication No. 5606/66, for example. These known proposals have been subject to several shortcomings. The unitary blocks are not very versatile; the mold that makes them is set-up to mold a block having a certain number of united caps in a row, no more and no less. This is satisfactory only where the user is content to have a set containing that particular number of writing instruments. The sets of conventional detachably secured caps have been subject to coming apart at inconvenient times. And the known cap sets have not had their individual caps adaptable to alternative use with both writing instruments filled with quick drying inks, which need to be sealingly capped when not in use, and writing instruments filled with pencil lead, non-water base ballpoint pen ink, fountain pen ink and the like, none of which require sealing caps and at least some of which can benefit from cap ventilation when not in use.